Re: [Boost-users] Copy a smart pointer into allocated memory

Thanks. I never used that placement new before. Always leaning... :) Also, thanks for the heads up about the destructor. I am destroying the weak_ptr() in the lua garbage collector. Tiago Alves Peter Dimov escreveu:

Tiago Alves:

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Hi, and sorry for the probable newbieness of this question!

I want to put weak_ptr into a piece of memory previously allocated.

this is my code:

// construct a weak_ptr from a share_ptr boost::weak_ptr<Obj>* cameraPtr = new boost::weak_ptr<Obj>(obj_share_ptr); // lua_newuserdata allocs memory and returns a pointer to it, just like malloc. boost::weak_ptr<Obj>* camera = static_cast(lua_newuserdata(pLuaState,sizeof(boost::weak_ptr<Obj>)));

Now i want camera to be a weak_ptr of ObjPtr. I tried different ways to do this, such as *camera = boost::weak_ptr<Obj>(obj_share_ptr); or camera->swap(*cameraPtr); or *camera = *cameraPtr;

I think none of these work because in fact they all do a swap() between camera and cameraPtr. Since camera is just an malloc, it will try to swap dirty memory and segfault sometimes (yes, sometimes this worked).

In C++, the way to construct an object of type X into uninitialized memory (pointed to by a pointer p) is called placement new:

#include <new>

new( p ) X;

In your case, you'd do:

void * p = lua_newuserdata( pLuaState, sizeof( boost::weak_ptr<Obj> ) ); new( p ) boost::weak_ptr<Obj>( obj_share_ptr );

The last line constructs a new weak_ptr in the memory at p from the obj_share_ptr argument (same as cameraPtr in your original example, but without the allocation.)

Later (probably in your 'gc' metamethod, whatever that is :-) ) you'll also have to dispose of the weak_ptr object; this is done by invoking the destructor:

p->~weak_ptr<Obj>();

In weak_ptr's case, you can also cheat and just reset() it, as its empty state consists of two NULL pointers and can be garbage-collected with impunity, but in principle, the proper way to end the lifetime of such an object is to call the destructor.